Modem wind-driven power-plants are predominantly designed to rest on tubular towers, in particular steel tube towers, because this design, termed shell design, is the simplest and most economical. Regarding large wind-driven power-plants having rotor diameters of more than 70 m and towers of heights in excess of 80 m, their output power being more than 1.5 megawatt, the critical engineering limitation is the required tower diameter at the tower base. Towers of diameters larger than 4.3 m can be transported only with difficulty because frequently the clearance underneath bridges would not allow going through the underpass. Moreover the total length and weight of such towers demands subdividing them into several tower sections that are bolted to each other by annular flanges. In addition to transportation costs, such large annular flange connection means entail considerable costs when very large wind-driven power-plants (3-5 Mw) are involved.
In view of the difficulties in transportation, concrete towers are used increasingly, being manufactured either at the erection site of the wind-driven power-plant or else consisting of smaller components that will be bonded and braced together. Both types of towers however entail higher manufacturing costs than tubular steel towers. As a result hybrids of steel pipes and concrete are sometimes built, of which the upper tower is as much as possible a steel pipe tower and only the lower tower segment, of which the diameter is too large for transportation, is made of concrete. However the transition zone between steel tower and concrete tower entails complex engineering and high costs.
Furthermore, lattice towers called power pylons up to 114 m high are already in use for large wind-driven power-plants up to 114 m high and allowing outputs up to 2 Mw. Besides the advantage of problem-free transportation, such towers however also have the critical drawback of a much larger horizontal expanse than a comparable steel tube or concrete tower, frequently raising the problem of the required safe distance between the rotor blade tip and the tower (blade clearance). If in the event of a storm, if the rotor blade were to be excessively bent out of shape, there would be danger of contact with the tower and dire consequences for the entire edifice.
On the other hand, the larger horizontal expanse of the lattice tower allows materials to be saved. This advantage is known in the construction industry and saves on total weight and hence lowers initial costs. On the other hand, this economic advantage is negated in general by the tower maintenance costs required over a service life of 20 years. Illustratively, the dynamically highly stressed towers of the wind-driven power-plants must be checked periodically, and such maintenance at the lofty heights of the lattice towers is dangerous, time-consuming, physically exhaustive and must be carried out by highly skilled specialists.
It is known from the German patent documents DE 736,454 and DE 198 02 210 A1 that the tower may comprise an upper and a lower segment, the lower one being a lattice tower and the upper one being tubular.
Such designs suffer from the drawbacks of requiring very demanding engineering work at the transition zone between the shell construction (tubular tower) and the framework construction (lattice tower). As a result, as regards extant lattice towers for wind-driven power-plants, in general only a tubular stub, the so-called “pot”, hereafter “stub”, will be inserted directly underneath the equipment nacelle to implement the transition to the equipment nacelle fitted with an annular flange. In such designs the transition is implemented in general by typically bolting four corner posts of the lattice tower by means of junction plates or the like directly and from the outside on the stub. This design works because the tower is directly underneath the nacelle and thereby experiences only relatively low bending forces. In such a design, substantially only the horizontal rotor thrust acting as a transverse force on the tower need be transmitted. Farther below, where the tower is primarily loaded by the rotor thrust bending torque generated by the leverage of the tower length, such a design becomes uneconomical.